What follows cannot possibly be considered real "advice" due to the fact that I really have no basis in expertise here. But as far as a free flow of intuition here goes:

With deck removed, mark the centerline of your deckbeam(s). Stick a piece of light cardboard onto your deckbeam , shear to shear. Use something that will allow you to remove it easily like spray matte. Mark the back of the cardboard with a line to match up with your centerline before sticking it on. Cut the top edge of the cardboard to the profile of the top edge of the beam.

Attach the deck, use duct tape or something to get it pulled down as close to final fit as possible. Using a long stick with one end set up with a marker and a pointy thing to act as a compass, ( the scribing type) scribe the deck's under surface on the cardboard. Also mark the cardboard at the point where the deck leaves the surface of the deckbeam, you should end up with two tickmarks, one one each side and (presumably) equidistant from center.

Now, remove the deck, remove the cardboard and slide it straight UP, keeping the centerlines matched until the intersection of (your tickmarks + the deck profile) reaches the top of your deck beam. Stick it back down.

Draw the deckbeam profile on the back of the cardboard.

If you cut out the cardboard at this line. I think you should end up with the profile of the piece you need to stick onto your deckbeam.

Ya think?

worth a try Jim